feat(plugins): allow message introspection (#6353)

This allows users to introspect messages (i.e. look at the elements they're made up of).

Reviewed-by: Mm2PL <mm2pl+gh@kotmisia.pl>

tests/lua/message/docs.lua

@@ -0,0 +1,48 @@
+-- These tests mirror the code example from the docs - make sure they work
+local tests = {
+    frozen_messages = function()
+        local my_msg = c2.Message.new({ id = "foobar" })
+        assert(not my_msg.frozen)
+        local chan = c2.Channel.by_name("mm2pl")
+        assert(chan)
+        chan:add_message(my_msg)
+        assert(my_msg.frozen)
+    end,
+    element_access = function()
+        local my_msg = c2.Message.new({
+            id = "foo",
+            elements = {
+                { type = "text", text = "foo" },
+                { type = "text", text = "bar" },
+                { type = "text", text = "baz" },
+            }
+        })
+        local elements = my_msg:elements()
+        assert(#elements == 3)
+        assert(elements[1].words[1] == "foo")
+        assert(elements[2].words[1] == "bar")
+
+        elements:erase(2) -- erase "bar"
+        assert(#elements == 2)
+        assert(elements[1].words[1] == "foo")
+        assert(elements[2].words[1] == "baz")
+    end,
+    append_element = function()
+        local my_msg = c2.Message.new({ id = "foo" })
+        local els = my_msg:elements()
+        assert(#els == 0)
+        my_msg:append_element({
+            type = "text",
+            text = "My text element",
+        })
+        assert(#els == 1)
+        assert(els[1].type == "text")
+    end
+}
+
+for name, fn in pairs(tests) do
+    local ok, res = pcall(fn)
+    if not ok then
+        error(name .. " failed: " .. res)
+    end
+end